From Quake Wiki
They don’t seal the map. To be compatible with existing Quake 1 mapping editors, detail brushes can be added by creating an entity with classname "func_detail". When qbsp reads the map file, it will add any brushes included in a func_detail entity into the worldspawn as details and remove the func_detail entity. Any number of func_detail entities can be used (useful for grouping) and all included brushes will be added to the worldspawn.
This classname used on brushes textured with fence textures will alpha-mask the #255 colored parts of that texture render those parts invisible.